Plasmas are widely used in the semiconductor industry for depositing thin films, for etching thin films and the underlying semiconductor material, and for dry-cleaning wafers. The plasma is typically formed in a cylindrical vessel or tube around which a wire is coiled. The reactant gases are introduced into one end of the tube and the atoms or ions that are generated by the plasma exit the other end of the tube and flow towards the wafer. Power is applied to the plasma by means of the coil. The mechanism by which the electrical power is transferred to the plasma can be either capacitive or inductive. Normally, both modes are present but one of the modes is predominant. In many applications the inductive mode is preferred because it produces a plasma having a higher ion density and because the ions in the plasma do not bombard the walls of the tube as much as they do when the plasma operates in the capacitive mode. This reduces wear on the tube and increases its life.
One structure for generating a plasma is described in U.S. Pat. No. 6,007,675 (FIGS. 2, 2B, 5A, 5B and 7) and U.S. Pat. No. 6,224,680 (FIGS. 2, 2B, 5 and 6). Both of these arrangements include a plurality of plasma tubes that generate plasmas used in a corresponding plurality of processing stages. As shown in FIG. 1, the coils are arranged in pairs 10, 12, and each pair of coils is connected to an RF generator 14 through a single impedance-matching network 16, which contains a phase angle detector 22, a control motor 24 and a variable capacitor 26. Phase angle detector 22 detects the phase difference between the signal produced by RF generator 14 and the signal in coils 10, 12 and actuates motor 24 and in turn variable capacitor 26 so as to drive the phase difference towards zero. As shown, coils 10, 12 are interconnected, and the coil around each of the tubes 18, 20 is interlaced, i.e., each coil consists of two windings, with the individual turns of each winding embedded between turns of the other winding. Ideally, the power supplied through the matching network 16 would be shared equally by the two tubes 18, 20.
In practice, it has been found that this structure has several defects. The plasma operates primarily in the capacitively coupling mode when the power supplied to each tube is less than about 500W. For example, when the plasma is used to strip bulk photoresist, it has been found that the coupling mode changes abruptly from capacitive to inductive when the power supplied to each tube increases beyond about 600-700W. This is shown in FIG. 2, which is a graph of ion density of the plasma versus power per tube. The ion density levels off at about 6.0E+11 ions/cm3 at 600-700W and increases rapidly above 750W, indicating that the coupling in this region becomes inductively coupling rich.
Moreover, in the arrangement shown in FIG. 1 the power supplied to the tubes 18, 20 is not in fact balanced. Typically, one of the tubes receives more power through normal perturbations, and this condition worsens as the power is delivered to the path of least resistance. Also, the reaction rate (e.g., the photoresist stripping rate) is limited by the available power.